You can upgrade Units by collecting duplicates of the same Unit. The amount of duplicates required to reach the next level is displayed on the bottom of the Unit's portrait. Once you have collected the required amount of Units, you can Upgrade the unit by:
  1. Tapping on the Unit you wish to Upgrade in the "Squad" tab.
  2. Then tap the "Upgrade" button below the Unit's portrait
Please note, upgrading Units cost Bucks. The amount of Bucks required to upgrade a unit increases with each level.

All the Best,
Hothead Games